The U.S. government appealed on Friday a federal court ruling that the gathering of Americans' telephone records by the National Security Agency of was likely unlawful.
U.S. District Judge Richard Leon in December criticized the agency's so-called metadata counter-terrorism program and said he could not imagine a more "indiscriminate" and "arbitrary invasion."
The Justice Department asked the U.S. Court of Appeals for the District of Columbia Circuit to reconsider Leon's opinion.
Another federal judge in Manhattan also ruled on the collection efforts last month, but found it lawful, calling it a "counter-punch" to terrorism that does not violate privacy rights.
The divergent opinions raised the prospect that the Supreme Court will need to resolve issues about the program, first disclosed by former NSA contractor Edward Snowden, who is now in Russia under temporary asylum.
President Barack Obama has defended the surveillance program, under which the government has collected millions of raw daily phone records, but has indicated a willingness to consider constraints. He is expected to set forth proposals later this month.